Select the auxiliary verb that completes the inversion:
Not until the satellite telemetry was restored _____ the mission controllers discover the space probe's actual trajectory.
Answer and explanation
Correct answer: A. did
- Step 1: Analyze the restrictive prepositional clause 'Not until...': an initial 'not until' clause triggers subject-auxiliary inversion in the main clause.
- Step 2: The main clause verb is the base form 'discover' in the past simple, requiring the auxiliary 'did': 'did the mission controllers discover...'.
Answer
did.
Why not the others: 'had' would require the past participle 'discovered'; 'was' cannot invert with an active transitive verb like 'discover'; 'would' changes the factual discovery into an ungrounded conditional prediction.
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